The invention relates to an inter-harmonic wave detection instrument based on a DSP (Digital Signal Processor) and a detection method of the inter-harmonic wave detection instrument. According to the structure of the inter-harmonic wave detection instrument, three-phase voltage and three-phase current are respectively connected with a filter circuit through a voltage transformer and a current transformer; the output end of the filter circuit is respectively connected with an over-zero comparator circuit and an A/D (Analog/Digital) sampling circuit; the output end of the over-zero comparator circuit is connected with a phase selection switch-on circuit; the output end of the phase selection switch-on circuit is connected with a phase locking frequency multiplier circuit; the output end of the phase locking frequency multiplier circuit is connected with the A/D sampling circuit; the output end of the phase locking frequency multiplier circuit is connected with a DSP chip. The inter-harmonic wave detection instrument can test inter-harmonic wave of a power grid, is precise and rapid in testing, can achieve an expected target, and functions as preventing equipment from damage caused by the inter-harmonic wave, and a reference is provided for processing on the inter-harmonic wave.